CENTRIFUGAL FAN -VIBRATION PROBLEM ANALYSIS

Introduction

In this blog we will discuss one live example of belt driven centrifugal fan, where we will discuss what parameters we will have to check and what solutions we must use to remove the malfunctioning of centrifugal fan unit.

We are using one centrifugal fan for forcing the heated air over the drying malt. During operation of centrifugal fan, our operation team are facing some malfunctioning of centrifugal fan. 

We will have to discuss the problems that operation team are facing and we will have to inspect the current condition of centrifugal fan and we will have to find out the root cause of malfunctioning of centrifugal fan. 

After identifying the centrifugal fan various components we will have to find out the corrective action and in order to avoid the repeated problems we must have to secure the preventive action.

Vibration analysis is a condition monitoring test which is widely applied for determining the operating condition of any rotating machine for example Centrifugal fan. In this report we will have to discuss problem of vibration for one Centrifugal fan with the help of the vibration analysis technique.

In this report we will mainly discuss the two points

1. Various causes of producing vibration in Centrifugal fan
2. Recommendation  to prevent recurrence

We are using one Centrifugal fan on foundation in order to force the heated air over drying malt. Specification of Centrifugal fan is as mentioned below.

Belt driven centrifugal Fan

Specification

• Diameter of impeller: 2.5 m
• Type of bearings : Spherical roller bearings
• Drive mechanism : Multiple v-belt drive

In our continuous process, our operation team was facing some issues and those issues are as mentioned below.

Operation issues

•      Pulsations, on closure of inlet vanes (flow control device)
•      Excessive vibration levels in spite of careful impeller balancing.
•      Prominent second and third harmonics in the frequency spectrum
•      When run down, irregular knocking noises produced.

Our maintenance team has investigated the issues and in first investigation, we have observed some issues as mentioned below.

• Failure of two 24 mm bolts holding steel pedestal to concrete foundation block
• Partial disintegration of grouting between pedestal & foundation
• Some damage to shaft surfaces where bearings attached

Causes of vibration in Centrifugal fan

There are various causes of vibrations in rotating machinery, let us understand the causes of vibration in centrifugal fan

1.    Bearing housing

In our first inspection we have observed the bearing housing / steel pedestal fasteners and we have noted the failure of two 24 mm bolts holding steel pedestal to concrete foundation block, due to failure of any foundation bolt of bearing housing or steel pedestal will lead the vibration because shaft rotates at very high RPM with the help of these bearing pedestal so if any foundation bolt will be loose or broken then vibration will be increased.

If any fastener will be in loose condition then in case of operation of centrifugal fan there will be smaller amplitude of vibration but due to continuous operation of centrifugal fan, above mentioned amplitude of vibration will be increased and it will increase the vibration and it will also affect the other fasteners.

After continuous operation of centrifugal fan, in a condition where the bearing housing / pedestal foundation bolts are in loosen situations, there will be heavy play between concrete foundation block and pedestal and fasteners will be broken due to continuous operation of centrifugal fan and continuous transmission of load .

2.    Bearing sheet

As we are well aware about the maintenance of bearings and if we are failing in maintaining the correct maintenance practice for bearings, we may observed the malfunctioning in bearing working efficiency. 

Each bearing will have its own working life hence if we are not inspecting the desired checking parameters of bearings , bearing might be damaged due to continuous operation of bearing under unplanned maintenance practice.

Once the bearing started malfunctioning it might be damaged in short period of time and once bearing got damaged such as its rotating elements got moved outward from the cage of bearing or its inner ring might be damaged. 

When bearing inner ring will be damaged or rotating elements will be damaged and still the centrifugal fan is in operation with the damaged bearings then it will lead the reduction of diameter at bearing sheet and also create under cut at bearing sheet where bearing will be fixed. 

Once the shaft bearing size will be reduced then in that condition there will be much more play between shaft and bearing and it will lead the condition where fasteners will be started in loosen condition and hence it will increase the amplitude of vibrations as we have discussed earlier.

3.    Concrete foundation

Partial disintegration of grouting between pedestal & concrete foundation block will lead the misalignment of bearing pedestal and once bearing pedestal will be misaligned then it will affect the rotation and working of bearings. 

Misaligned bearing pedestal, which is due to basically disintegration of grouting between pedestal & concrete foundation block, will lead the malfunctioning of bearings and bearings might be damaged in coming working of centrifugal fan. 

As we have discussed the malfunctioning of bearing will lead the damaging of fasteners of housing / pedestal. Hence we may conclude that if one component of unit is damaged then it will affect the working of other components.

4.    Motor foundation bolts

If motor foundation- fasteners are also in loose condition then it will lead the vibration in motor unit and hence other foundation bolts will also be loosen and continuous operation of centrifugal fan under the loose bolts of motor foundation will lead the misalignment of motor pulley and driven pulley .

If we will continuous use the centrifugal fan in misalignment situation then it can also cause the bending of driven shaft if shaft material hardness is lower than 35 HRC. If driven shaft will be bend then we may also observe much vibration If centrifugal fan driven shaft rotates eccentrically due to bending. 

Driven shaft will rotates eccentrically only if there is some deflection in driven shaft or we may say that if shaft is slightly bend.

We must noted it that if shaft is bend during operation or by any way , we will not able to amend and straighten the shaft completely i.e. 100 % straighten could not be achieved if shaft is bend hence the better solution is to replace that shaft in spite of using amended shaft .

5.    Pulsations, on closure of inlet vanes

When inlet valve will be closed, there will be shocking impact over impeller blade and it may cause vibration as after shocking impact, load will be transferred over foundation bolts; shaft and impeller blades and this shocking impact load may damage the fasteners and can cause vibrations.

6.    Excessive vibration levels in spite of careful impeller balancing

If impeller is balanced, still we are observing the vibration then in this case we have only that reason as we have discussed earlier and there might be problems of pulley misalignment.

7.    Gear box oil level

If we are also using gear box in our centrifugal fan unit for giving higher torque with suitable speed, we will have to ensure about the efficient working of gear box also. Gear box consist bearings, gears, shafts, gear oil 320 and seals. 

Hence if any gear teeth will be damaged or bearing will be damaged, it will produce abnormal sound and also produce some vibrations and this vibration will be transmitted to other component of our unit.

8.    When run down, irregular knocking

When run down, irregular knocking will be observed from unit only if gear box gear backlash will be increased or mechanical connections are not perfect.

Recommendation to prevent recurrence

Vibration problem is quite dangerous for our smooth running of machinery.
We will have to find out the one time solution for avoiding the frequent problem of vibrations.

Bearing Housing Foundation bolts

Bearing housing foundation bolts looseness might increase the vibration hence we will have to avoid looseness of fasteners. In order to avoid the frequent looseness we will have to check following points.

• Foundation fasteners must not be undersized.
• Foundation fasteners must have with plain washer  as well as spring washer of correct size.
• Bearing housing alignment must be inspected so that each fasteners should be in center.
• Fasteners must have one lock nut for permanent solution of frequent bolts loosen situations.
• Routine inspection of fasteners for avoiding the problems of vibrations.

Bearing

Bearing must be inspected on routine basis for avoiding the failure of bearing because if bearing rotating element and inner ring will be damaged then it will deteriorate the bearing sheet of shaft and it will lead the vibration of unit. 

In inspection of bearing we will have some parameters that we will have to check for determining the working condition of bearings and these parameters are as mentioned below.

• Bearing housing foundations bolts
• Bearing temperature
• Bearing lubrication quality
• Clearance determination of bearings
• Any abnormal sound from bearing
• Bearing inner ring and rotating elements and cages
• Play between bearing inner ring and shaft size where bearing will be fixed

Bearing must be lubricated on routine basis in order to secure the efficient operation of bearings.

Concrete foundation

Concrete foundation must not be cracked and must be parallel so that it may not help for producing the vibrations. Because if any cracked will be observed then in that situation it will affect the bearing and hence further it will affect the foundation fasteners.

In order to avoid the cracking of concrete, we will have to maintain other parameters in efficient working condition so that there will not be any vibration because concrete will be cracked due to vibration.

If misaligned occur then in that condition shaft will be deflected due to continuous operation of centrifugal fan unit under misalignment condition we will have to replace the shaft in spite of using the same amended shaft as we can not amend shaft completely. New shaft should have some important characteristic for avoiding the problem of bending of shaft.

• Material of shaft must be EN 24 for providing the required hardness about 50 to 60 HRC
• Shaft must have stress concentration factor and sharp steps must be avoided , we will have to use the shaft with steps with radius of 3 mm instead of sharp steps.

Pulsations, on closure of inlet vanes

In order to avoid the shocking impact, we must have to close or open inlet valve slowly, this could be done by operation team after operation training.

Gear Box

As we have discussed that gear box consist gear, shafts, bearings, oil and seals. we will have to inspect oil level on daily basis as if gear box oil level is low then it will be heated due to excessive friction between meshing teeth of gears.

If gear box oil level is high then in that case also gear oil temperature will be high and it may damage the seals and once seals will be damaged then oil will flow outward from gear box. Hence, we will have to inspect gear box with following parameters on daily basis.

• Gear box oil level
• Gear teeth condition
• Oil leakage from seals
• Abnormal sound
• Gear box temperature
• Excessive vibrations
• Gear box foundation bolts
• Gear box alignment
•  Bearings conditions

Abstract

We have concluded following points after discussing the current situation.

• Single problem will lead the multiple problem in centrifugal fan unit
• A single loose bolt may lead the damaging of complete centrifugal fan unit
• We must concentrate the each problem and we must have to resolve the problem without losing time as one problem will lead the creation of other problems , hence in order to avoid the breakdown of unit we will have to resolve problem without any delay
• We have also studied that there are some standards maintenance practice that we must  have to follow for efficient operation of unit

 Keep reading .....

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